Anti-reflective coatings have been used in various applications for some time. Exemplary applications include lenses, glazing units, mirrors and the like. It is becoming desirable to use anti-reflective coatings on architectural and automotive glazing units, especially on the inside and/or outside surfaces of motor vehicle windshields. A suitable anti-reflective coating on the inside surface of a motor vehicle windshield would facilitate the use of lighter colored instrument panel materials. Without an anti-reflective coating, vision through the windshield might be impaired by light from the upper surface of such lighter colored instrument panel reflecting on the inside surface of the windshield. An anti-reflective coating on the outside of a windshield increases transmitted light intensity and helps meet applicable minimum transparency requirements. Presently, minimum transmittance of visible light for motor vehicle windshields is 70% in the United States and 75% in Europe. Therefore, to be suitable for use in a vehicle windshield or other glazing application, the anti-reflective coating must not reduce the transparency of the glazing unit to an unacceptable degree.
Numerous anti-reflective coatings are known, many of which comprise a film stack in which a first film of relatively high refractive index material is paired with a second film of lower refractive index material. Thus, for example, U.S. Pat. No. 4,846,151 to Simko, Jr. suggests that various surfaces of transparent plates used in solar collectors can be coated with an anti-reflective material. Exemplary materials are listed, including multi-layer coatings such as silicon dioxide paired with aluminum oxide or titanium dioxide. Similarly, U.S. Pat. No. 4,822,748 to Janesick et al suggests the use of an anti-reflective coating on glass used in picture frames and the like. Specifically, it suggests the preparation of a triple layer film stack in which a film of titanium oxide is sandwiched between films of silicon dioxide. Other materials, such as zirconium oxide, tantalum oxide and magnesium oxide also are mentioned. The use of silicon monoxide is suggested as an anti-reflective coating for optical parts made of synthetic resin in U.S. Pat. No. 4,497,539 to Sakurai et al. Silicon monoxide also is suggested, as is silicon dioxide, as an anti-reflective layer having high infrared reflectivity and high visible light transmission suitable for use in heat-mirrors in U.S. Pat. No. 4,822,120 to Fan et al. In U.S. Pat. No. 4,815,821 to Nonogaki et al a light transmitting glass panel is suggested having on its surface a coating consisting of a silicon monoxide layer over a titanium oxide layer. The silicon monoxide layer is said to be intermittently spaced from the titanium dioxide layer by a light absorbing layer of colloidal carbon. A transparent optical article, such as a lens, is suggested in U.S. Pat. No. 4,765,729 to Taniguchi. Silicon dioxide is suggested as a suitable anti-reflective coating for the surface of the article.
The use of an anti-reflective coating on both the inside and the outside of an ophthalmic lens is suggested in U.S. Pat. No. 4,070,097 to Gelber. Each of the two coatings is said to have two layers, a dielectric layer and a metal layer. For the metal layer, suitable materials are said to include nickel, chromium, Inconel and Nichrome (a material comprised essentially of nickel and chromium). The metal layer is said typically to have a thickness ranging from 10 to 40 Angstroms. Various materials, including silicon dioxide, are listed for the dielectric layer. A second U.S. patent to Gelber, U.S. Pat. No. 3,990,784, is directed to coated architectural glass having a multi-layer coating on its surface. The coating is said to comprise first and second metal layers spaced from each other by a dielectric layer disposed between them. An additional metal oxide layer is said to be used optionally for anti-reflective purposes. Nickel is mentioned as being a suitable metal together with silicon dioxide as the dielectric layer.
The optical properties of silicon/silicon dioxide multilayer systems are discussed in Stone et al., Reflectance, Transmittance and Lost Spectra of Multilayer Si/SiO.sub.2 Thin Film Mirrors and Antireflection Coatings For 1.5 .mu.m. Applied Optics, Vol. 29, No. 4 (1 Feb. 1990). Stone et al suggest that in the spectral region between 1.0 and 1.6 .mu.m, a useful and easy to handle combination of paired layers is silicon and silica. The paper is directed to the fabrication of multilayer systems. It is noted therein that the greater the difference in the index of refraction of the paired layers, the fewer the number of layers will be needed to obtain a desired level of reflectance. Silicon is noted to have a relatively high index of refraction. The paper states that silicon cannot be used as a material in the film pair for light below about. 1.0 .mu.m wavelength, for visible light, for example, due to its high absorption of light in that range. Visible light has a wavelength in the range of about 0.4 to 0.75 .mu.m. Thus, while suggesting that a simple two layer anti-reflection coating can be made using silicon and silicon dioxide, the article clearly teaches that such anti-reflection coating is not suitable for applications requiring transparency to visible light. The article notes that Si/SiO.sub.2 film pairs for high reflectance mirrors and anti-reflection coatings have been deposited by reactive sputtering. The coatings discussed in the paper are said to have been deposited by electron beam evaporation onto glass substrates. The anti-reflection coatings described in the Stone et al article are said to consist of a layer of silicon about 150 Angstroms thick with a layer of SiO.sub.2 thereover having a thickness selected to yield minimum reflection. A silicon layer of that thickness is substantially opaque to visible light and reflectance percentage is shown in the paper only for light far above the visible wavelength range. For a layer of silicon of that thickness, a SiO.sub.2 layer of about 2800 Angstroms is employed by Stone et al. It is further stated that the minimum reflectance value is not very sensitive to the thickness to the silicon layer over a thickness range between 75 and 200 Angstroms. Even at the low end of this thickness range, however, the layer of silicon would be substantially opaque to the visible light component of ordinary sunlight.
Similar teaching is presented in Pawlewicz et al., 1315 nm Dielectric Mirror Fabrication By Reactive Sputtering presented at the Topical Meeting on High Power Laser Optical Components held at Boulder, Colo. on Oct. 18-19, 1984. Low levels of light absorption are reported in that paper for five reactively sputtered amorphous optical coating materials, including a Si:H/SiO.sub.2 film pair. The low absorption was measured for light in the 1.3 .mu.m range and it is taught in the conclusion of the paper that the Si:H material is not useable at visible wavelengths. The same point is made in Pawlewicz et al., Optical Thin Films-Recent Developments In Reactively Sputtered Optical Thin Films, Proceedings of the SPIE, Vol. 325, pp. 105-112 (Jan. 26-27, 1982). Table 1 of that paper lists light wavelengths of 1,000 to 9,000 nm (1.0 to 9.0 .mu.m) as the range for which optical coatings of silicon are useful. Thin film coatings of Si.sub.1-x H.sub.x for reducing light absorption of infrared laser wavelengths 1.06, 1.315 and 2.7 .mu.m are discussed in Pawlewicz et al., Improved Si-Based Coating Materials for High Power Infrared Lasers (November, 1981).
The optical properties of Si:H are discussed also in Martin et al., Optical Coatings for Energy Efficiency and Solar Applications, Proceeding of the SPIE, Vol. 324, pp. 184-190 (Jan. 28-29, 1982). The effect is discussed of hydrogen content and Si:H bonding on various optical properties at 2 .mu.m, a non-visible wavelength. Multilayer Si:H/SiO.sub.2 laser mirrors with reflectance greater than 99% at non-visible wavelengths 1.315, 2.7 and 3.8 .mu.m also are described. The article notes that Si:H/SiO.sub.2 multilayer coatings are easily fabricated by sputtering, since only a single Si target is required, with either H.sub.2 or O.sub.2 being introduced into the sputtering chamber to form Si:H and SiO.sub.2 layers, respectively. The high absorption coefficient in the visible region is said to make thin films of Si:H suitable for use in solar cells to absorb solar radiation.
Various glazing product needs would be met by a new anti-reflective coating system which is substantially transparent to visible light and which can be deposited onto a substrate surface by economical and industrially feasible techniques. In addition, certain glazing applications, such as the above mentioned inside surface of a motor vehicle windshield, require relatively hard and durable anti-reflective coating systems. It is an object of the present invention to provide an anti-reflective coating system, or a glazing unit having an anti-reflective coating thereon, which meets one or more of these product needs. Additional features and aspects of the invention will be understood from the following disclosure and description thereof.